1. Field of the Invention
The present invention is generally concerned with variable speed drives of the type employed for the controlled driving of any type of member from any type of motor and, in an automobile vehicle, for example, to drive any auxiliary member of the latter, such as an alternator, water pump, compressor or fan as usually fitted to the vehicle, from the motor of the latter.
The invention is more precisely concerned with variable speed drive pulleys of the kind comprising a hub, two annular flanges around the hub in axial face-to-face relationship to one another, at least one of which, hereinafter referred to as the mobile flange, is movable relative to the hub in the axial direction, and elastic control means responsive to centrifugal force and operative on the mobile flange so as to reduce the distance between the flanges when their rotation speed increases.
2. Description of the Prior Art
Pulleys of this kind are described in particular in French Pat. No. 2 347 578 filed Apr. 9, 1976 under registration number 76 10392, and in French Patent Application No. 81 21590 filed Dec. 18, 1981.
These documents describe variable speed drives comprising a driving pulley and a driven pulley rotationally coupled by an endless belt. These variable speed drives are intended in particular to drive auxiliary equipment of an automotive vehicle, providing for a drive ratio which tends to decrease when the rotation speed of their driving pulley increases. To this end this driving pulley comprises, around a hub, two frustoconical annual flanges of which one at least is movable axially and which are adapted to move apart when their speed increases. The complementary driven pulley comprises, around a hub, two frustoconical annular flanges one at least of which is mobile axially and which are adapted to move closer together when their speed increases. In this type of variable speed drive it is the driven pulley to which the invention is of particular relevance.
It should be noted that in the aforementioned patents control means tending to move the two flanges closer together or further apart, as appropriate, are formed by a frustoconical diaphragm spring comprising radial fingers to which are attached flyweights responsive to centrifugal force.
Thus, as is known, in a device of this kind the curve representing variations in the speed of the driven pulley as a function of motor speed comprises a first portion in which the speed increases, a second portion in which the centrifugal action of the flyweights causes the transmission ratio to be reduced and a third portion in which the variable speed drive ceases to act and the transmission ratio remains constant.
In practice the second portion corresponds to a substantially constant speed of the driven pulley, independent of the speed of the driving shaft. This substantially constant (or at least only slightly varying) speed of the driven pulley is generally employed to drive auxiliary equipment at the optimum speed, for a broad range of instantaneous speeds of the driving pulley. By way of example, in the case of an internal combustion engined automobile vehicle the variable speed drives conventionally employed are adapted to maintain the speed of the driven pulley at around 1,500 rpm, whereas the speed of the driving pulley fluctuates between 1,000 and 2,500 rpm.
The present invention is specifically concerned with the transition between the second and third portions of the aforementioned curve, representing relative variations in the speeds of the driving and driven pulleys. This transition corresponds to the moment at which the flanges of the driving pulley reach their configuration of maximum separation and at which the flanges of the driven pulley reach their configuration of minimum separation.
In practice, one of the problems raised by designing variable speed drive pulleys resides in the geometrical design of the extreme configurations of the various pulleys, taking into account notably the fact that the extreme configurations of the driving and driven pulleys of the same variable speed drive must be compatible, which means that they must produce a tension in the belt linking the pulleys which is sufficiently high to ensure good adhesion between the belt and flanges and thus proper transmission of torque, but sufficiently low to ensure a long service life of the belt. The belt tension must be maintained within relatively closely spaced limits.
In currently known variable speed drives the extreme configurations of the pulleys at high speeds result from a compromise between the forces exerted on the mobile flange by the belt by virture of its tension, on the one hand, and by the diaphragm spring which urges the flange towards an idle configuration, on the other hand, this latter force being tempered by the action of the flyweights. Because the tension in the belt is taken into account, the compatibility condition previously mentioned is achieved.
It is sometimes advantageous to use a "working point" in the third portion of the curve, at increasingly higher speeds.
At these higher speeds the flanges of a driven pulley tend, because of the action of the centrifugal flyweights, to move closer and closer together and therefore to move the associated transmission belt further and further outwards in the radial direction, resulting in excessive tension in the belt prejudicial to its service life.
An object of the present invention is to limit the centrifugal effects on the belt around a pulley of the aforementioned type at high speeds without cancelling the centrifugal effect, which would be prejudicial to correct operation of the variable speed drive.
Thus precise control and appropriate behavior of a variable speed drive pulley are achieved at the end of the movement towards one another of its flanges under the effects of centrifugal force.